1. Field of the Invention
The present invention relates to radio communications apparatuses with multiple antennas, radio communications methods, and radio communications systems.
2. Description of the Related Art
A base station in a radio communications system that has multiple transmit antennas can precode a transmit signal to improve received signal quality at a mobile station. This type of scheme is disclosed in Non-patent document 1.
Generally, the precoding is performed by applying, in advance to the transmit signal, an inverse characteristic H−1 of a propagation path characteristic H between the base station and the mobile station. When there are multiple transmit antennas, directivity can be adjusted using the precoding. Therefore, the information on the propagation path characteristic H must be obtained by the base station in order to precode the transmit signal at the base station. More specifically, for a frequency division duplexing (FDD) system, the propagation path characteristic is not the same between uplink and downlink. Therefore, there is a need for the base station to transmit a pilot signal to the mobile station, for the mobile station to estimate the downlink propagation path characteristic based on the pilot signal, and for the mobile station to report, to the base station, information on the estimated propagation path characteristic. The greater the number of transmit antennas at the base station, the greater an improvement effect due to precoding can be expected. However, as the number of transmit antennas increases, the number of propagation paths to be estimated increases, causing a need to increase the number of pilot signal types.
FIG. 1 shows an exemplary signal configuration for transmitting a pilot signal from multiple transmit antennas. This type of exemplary signal configuration is disclosed in Non-patent document 2. FIG. 1 illustrates how the pilot signal is mapped for the numbers of transmit antennas at a base station of 1, 2, and 4. In the exemplary signal configuration in FIG. 1, it is anticipated to use an orthogonal frequency division multiplexing (OFDM) scheme. In the example shown, all of the pilot signals transmitted from the respective transmit antennas are arranged such that they do not overlap in both time and frequency domains. As a result, as the number of transmit antennas used by the base station increases, the number of pilot signals increases as well. For example, for a case of the number of transmit antennas at the base station being 2, the number of pilot signals within a frame doubles relative to a case of the number of transmit antennas being 1; and for a case of the number of transmit antennas being 4, the number of pilot signals triples. In other words, as the number of transmit antennas at the base station increases, a proportion of pilot signals in a transmit frame (overhead) increases, causing a decrease in radio resources for a data signal, etc.
Now, in this type of radio communications systems, automatic repeat request (ARQ) or hybrid automatic repeat request (HARQ) is performed, and, if an error is detected in a received packet, the packet is retransmitted. In a related art radio communications system, even in cases of retransmission, a pilot signal is transmitted, in all of the cases, using a configuration as shown in FIG. 1.
Therefore, there is a problem that, when the number of transmit antennas are great, overhead in a transmit frame becomes significantly large. Such a problem is of concern in uplink as well as downlink.